Carbon/carbon composites include those structures formed from a fibrous reinforcement which itself consists principally of carbon and are densified for enhanced strength employing principally carbon. Such carbon/carbon composites typically are characterized by high strength, low density, and high modulus. Carbon/carbon composite materials are eminently useful in applications where high temperature strength and high strength-to-weight ratio are important. Such carbon/carbon composites may be formed, for example, by impregnating graphite fiber cloth with resinous material, layering plies of the resin impregnated graphite cloth, curing the resin under pressure and thereafter carbonizing the resin impregnated layer plies with application of heat and pressure. Densification may also be accomplished by CVD (chemical vapor deposition) or CVI (chemical vapor infiltration) of carbon or by impregnation with a resin or pitch followed by carbonization.
An important limitation to the use of carbon/carbon composites is their susceptability to oxidation in high temperature oxidizing environments. Oxidation not only attacks the surface of the carbon/carbon composite, but also enters pores that invariably are present in such structures oxidizing the carbon fibers adjacent to the pores and surfaces of the pores, and thereby weakening the composite. Such oxidative degradation is of particular concern when operating temperatures will exceed about 500.degree. C. (932.degree. F.).
Much effort has been and continues to be directed toward identification of methods and mechanisms for reducing or eliminating oxidative weight loss and accompanying loss of strength associated with operation of carbon/carbon composites at elevated temperatures.
One approach has been to reduce opportunities for intimate or permeating contact between the carbon/carbon composite and any oxidizing environment in which such structure exists or is operated. One mechanism for reducing such contact has focused upon sealing of the exterior surfaces of a carbon/carbon composite thereby forestalling or foreclosing entry of oxidizing agents thereinto.
It has been proposed that exterior surfaces of a carbon/carbon composite be coated with a ceramic material such as silicon carbide to prohibit entry of oxidizing agents, typically molecular or ionic oxygen from the atmosphere, into such a carbon/carbon composite. Such ceramic coatings have provided only limited success as they have been characterized by a tendency to crack as a result of thermal cycling of the coated structure. Such cracking is believed to be due to differences in coefficients of thermal expansion of the carbon/carbon composite and the ceramic coating.
It has also been proposed that exterior surfaces of carbon/carbon composites be coated with a single or multi-component glass forming seal coat such as boron or boron plus zirconium-containing substance tending to form a glass or glass-ceramic material which is to a degree flowable and amorphous in character at temperatures (e.g. 649.degree.-1316.degree. C. or 1200.degree.-2400.degree. F.) associated with the environment in which it is desired to protect the carbon/carbon composite from an oxidizing environment. Such glass forming substances have been observed to inadequately protect the carbon/carbon composites when the temperature is cycled between ambient and about 649.degree.760.degree. C. (1200.degree.-1400.degree. F.), a temperature range in which carbon/carbon composites are particularly susceptible to oxidation. A possible explanation for the somewhat disappointing performance of such glass forming materials at these temperatures lies in t he tendency for the formed glasses at those temperatures to be more solid than flowable. At such temperatures, these glasses are more susceptible to cracks due to a significant differential between the coefficient of thermal expansion for such glasses and the coefficient of thermal expansion for the underlying carbon/carbon composite.
Borate glasses have been proposed for protection of carbon/carbon composites. A serious drawback for many B.sub.2 O.sub.3 glasses is degradation by hydrolysis and leaching or evaporation under proposed operating conditions which include humid weather. The viscosity of such glasses may change widely with increasing temperature. Viscosity stabilized boron oxide glasses suitable for protection of carbon/carbon composites from oxidation are described in U.S. Pat. No. 4,795,677 to Gray, the disclosure of which is herein incorporated by reference. These glasses have proven effective in protecting carbon/carbon composites from oxidation in dry environments. Efforts to improve resistance to hydration and subsequent volatilization and/or leaching have not been entirely successful. Modified borate glass compositions, have exhibited carbon wetting and viscosity characteristics that represent significant compromises compared to borate glasses not modified in such a manner.
Accordingly, it is a general object of the present invention to provide carbon/carbon composites that have improved resistance to oxidation even when exposed to a moisture-containing environment.